1. Field of the Invention
The most frequently used printing plates for the offset printing process usually consist of a substrate on which a photosensitive layer is firmly applied. This layer is exposed to light, after which the nonimage part must be removed completely from the surface. By means of the hydrophobic layer which remains behind (image part), ink can be applied to the product to be printed, which however is ensured only when water is present in the region of the nonimage parts. The wettability with water (hydrophilic property) in the region of the nonimage parts is of decisive importance for a high-quality printed image. It is known that alumina has such properties.
2. Description of the Related Art
It is therefore obvious to use aluminum substrates, to roughen the latter in order to improve anchoring of the printed image and to oxidize the surfaces. Chemical or electrochemical processes, also in combination with mechanical processes, for roughening pure aluminum have been disclosed, for example, in DE-A-34 13 899.
The multistage processes require a uniform aluminum composition at the substrate surface in order to ensure that, in a controlled chemical process, a uniform surface topography free of grains is obtained. The disposal of the baths and of the resulting solid are to be regarded as negative factors.
German Auslegeschrift 1,300,579 has disclosed a process in which a plasma is generated by means of an electric arc between a heat-resistant electrode and a metallic substrate in an inert gas mantle, with the aid of which plasma printing plates are roughened with small amounts of waste, and the surface can be modified by adding materials in such a way that it has improved hydrophilic properties. However, this process is difficult to realize in practice since it is very dependent on the intensities of the transmitted arcs, which are determined by a plurality of factors.
German Auslegeschrift 2,348,717 has disclosed a further process for applying damping solution-conveying layers to printing plates for the offset printing process. Layers of sparingly soluble or insoluble carbonates, silicates or quartz are provided, said layers being applied to roughened substrates by the plasma-spraying process method and then being ground to produce the suitable roughness. The image area is obtained by partially removing the coating. However, this process is very expensive owing to the machining and the etching process for removal of the layer.